I think Clinton's speech will have more of an impact on a DNC bounce than Obama. He's probably the only former president with any credibility on the economy. Carter is liked for his humanitarian efforts but not well-renowned as a president, Bush Sr. isn't really in the limelight, and Dubya is... well.
There are a lot of moderates - Democrats, Independents, and Republicans alike - who would rather have Clinton back, and he wholly embraced the president and his policies. It wasn't a token "Support the Democrat, wink wink" thing, either.
Well for one there'll probably be more democrats in the House regardless of if they take it, so Boehner will have less wiggle room with his own caucus and if he wants to pass ANYTHING he'll have to deal with Pelosi. I'm not very optimistic though, I think they might pass a few smaller, low-hanging-fruit bills like the DREAM Act but as far as anything big, it'll be mostly gridlock.
Which is why, of course, Democrats need to win the House back. I just hope that Princeton model is accurate.